This invention relates to multi-speed drive transmissions for vehicles or the like and more particularly to fluid operated control systems for shifting transmissions between drive ratios in response to movement of an operator's shift lever or the like.
Prior U.S. Pat. No. 3,508,450 discloses a highly advantageous form of transmission in which a shift is effected by disconnecting the internal change speed gears from both the driving engine and the wheel drive line of the vehicle and then temporarily braking the gears to a stationary or near stationary condition so that disengagements and re-engagements are readily accomplished without requiring synchronizers or the like. After re-engagement of the gears to establish a newly selected drive ratio, the brakes are released and input and output clutches re-engage to re-establish drive through the transmission. Another prior U.S. Pat. No. 3,570,636 describes a fluid circuit for automatically cycling the various pneumatic devices in a transmission of this kind to accomplish a selected shift.
Utilizing the control system of prior U.S. Pat. No. 3,570,636, the operator need only move the shift lever from one position to another to effect a shift. The necessary timed actuations and de-actuations of the several pneumatic devices including input and output clutches are then sequenced automatically by the control system. The structures of the above-identified prior U.S. patents combine the compactness, economy and basic efficiency of so-called standard or manual transmissions with the superior shifting performance and much of the ease of control of so-called automatic transmissions. In addition, the absence of synchronizers enables realization of a large number of drive ratios in a transmission having a small number of change speed gears.
Another prior U.S. Pat. No. 3,667,309 discloses an improved transmission of the general type discussed above in conjunction with an improved pneumatic control system. In the structure of prior U.S. Pat. No. 3,667,309, braking of the change speed gears at the start of the shift is accomplished with brakes that stop rotation of the input and output elements of the gearing section. At the conclusion of the shift, the input brake is released slightly in advance of release of the output brake so that any gear teeth which have failed to engage because of tooth abutment are enabled to engage while turning at low speed and under light loading. Other advantageous improvements to transmissions of this general type and to control systems therefor are described in prior U.S. Pat. Nos. 3,667,583; 3,620,097; 3,570,541; 3,744,608; co-pending application Ser. No. 369,714 of Kenneth R. Moline et al, filed June 13, 1973 now U.S. Pat. No. 3,863,520; and co-pending application Ser. No. 372,331, of Philip S. Webber et al filed June 21, 1973, all assigned to the assignee of the present application.
In shifting a transmission of the general type discussed above, there is a possibility that change speed gears which are to be engaged may come to rest in a position at which gear teeth to be engaged are abutted and engagement is temporarily blocked. Certain of the above-identified patents teach means for resolving this problem by transmitting a limited torque to the gears, to bring teeth into the necessary relationship for engagement, prior to the re-establishment of drive through the transmission. While this is generally effective for the desired purpose, full torque is applied to the gears after a predetermined time and severe tooth wear from skating or grinding can then occur if the desired engagements have not been completed for some reason.
Considering still another characteristic of prior transmissions of this kind, the shift control system must undergo certain preliminary changes to be in condition for a shift cycle. For example, a lock which inhibits movement of the operator's shift lever between drive ratio positions must release. As heretofore constructed, this lock has not released the shift lever for movement to another drive ratio setting and certain other conditioning operations have not been initiated until the operator made a small sideward movement of the lever prior to movement to a new drive ratio setting. A period of time is required to release the lever and for the control system to condition itself for shift. During this period the operator cannot complete the control lever movement necessary for the initiating of the shift. While this delay is small, typically a fraction of a second, it is disturbing to the operator and has an appreciable effect on the operation of the transmission. Power is not delivered to the vehicle wheels during the shift period. Any prolongation of the shift period results in a loss of engine output which would otherwise be available to do useful work.
Considering still another characteristic of prior transmission of this general form, a high risk of component damage is present if gear couplings required to disengage in a directional shift, i.e., from forward drive to reverse drive or vice versa, should fail to disengage for some reason such as torque locking. At the conclusion of such a shift certain portions of the gearing system might then be attempting to transmit drive in a forward direction while others are attempting to transmit torque in a reverse direction. Prior transmissions and control systems have not included any fully reliable and positive means for guarding against such an occurrence.